1. Field of the Invention
The present invention relates in general to land-based vehicular navigation apparatus and in particular to a method and apparatus comprising the combination of a relative positioning system (RPS), such as a vehicular dead reckoning navigation system with map-matching and an absolute positioning system (APS), such as a Loran-C system or a Global Positioning System (GPS), the latter systems being used to automatically reposition and recalibrate the RPS as required.
2. Description of Prior Art
Generally, there are several methods and systems used to navigate a vehicle over land, each with its advantages and disadvantages. Examples include a relative positioning system (RPS), such as a dead reckoning system or a dead reckoning system with map matching, and an absolute positioning system (APS), such as a Loran-C system or a Global Positioning System (GPS).
A conventional dead reckoning system with map matching, such as disclosed in U.S. Pat. No. 4,796,191, entitled Vehicle Navigational System and Method, and assigned to the assignee of the present application, has a number of advantages. It can operate in a fully self-contained way, requiring no equipment outside the vehicle in which it is used. It typically has high accuracy over significant intervals of time. It is linked to an electronic map of roads which can automatically eliminate minor vehicular position errors and measurement noise and provide a graphical user display. For example, as a vehicle using such a system moves, onboard wheel sensors, a magnetic compass and/or other sensing means computes the vehicle's position using dead reckoning techniques. The computed position is compared frequently with an electronically stored map of roads. If the computed position does not correspond to a location on the nearest appropriate road, the system automatically corrects the vehicle's position to place it on that nearest road.
The above-described dead reckoning navigation apparatus, however, has a number of disadvantages. One of the disadvantages is that sometimes navigation performance can degrade if the map matching relocates the vehicle's position to an incorrect road. This can occur because of an extreme anomalous magnetic field, wheel slippage or map errors. Another disadvantage arises if the difference between the computed vehicle position and the nearest appropriate road is too large, i.e. exceeds a predetermined allowable error estimate. Under these circumstances the dead reckoning system will not update its position. Once an incorrect update has been made or the errors become too large, precision navigation may not be automatically regained without manual intervention.
Another disadvantage of conventional dead reckoning navigation apparatus is that it typically requires that the operation of the system be visually monitored by the operator and manually calibrated and that, after calibration, correct initial position information be entered manually.
Some absolute navigation systems, such as those based on reception of Loran-C or GPS signals, have the advantage of providing high precision, at least some of the time, the ability to regain high precision position information after the loss of a signal, the ability to provide correct initial position information and the capability to be automatically calibrated.
Such systems, however, also have a number of disadvantages. For example, signal dropouts can leave a vehicle without any navigation information beyond the position computed before the dropout. There can be an offset, such as imposed by the selective availability of GPS, as well as transmitter and receiver clock timing differences and the like which can be quite large, e.g. hundreds of meters in the case of Loran-C. Moreover, there is no link to a map database. Thus, even if positions are plotted on a map electronically or otherwise, measurement errors, e.g. jitter, will be apparent. Also, without a link to a map database, no link to a road network is available to provide path computations and advanced user interfaces.